Sulfite pulping process with urea



United States Patent 3,368,935 SULFITE PULPING PROCES WITH UREA AlfredM. Heald and Julius D. Robertson, Hartsville, S.C., assignors to SonocoProducts Company, Hartsville, S.C., a corporation of South Carolina NoDrawing. Filed Oct. 12, 1964, Ser. No. 403,384 7 Claims. (Cl. 162-72)This invention relates to a pulping process and more particularly to aprocess for digesting cellulosic raw material such as wood chips by thesulfite process.

The pulping of cellulosic raw materials such as wood chips byconventional sulfite pulping processes has always been characterized bycertain drawbacks which continuous efforts have failed to overcome.While many solutions have been proposed for the elimination of thesedrawbacks, efforts to date have culminated in procedures which, ratherthan eliminating these drawbacks, serve merely to compensate in part forsuch drawbacks and, at the same time, are expensive and time consuming.For instance, in a conventional sulfite digestion of wood chips using anacid sulfite or bisulfite cooking liquor, the initial pH of the liquoris generally within the range of approximately 1.5 to 4.5. Utilizingsuch a low pH during the initial stages of digestion has the advantageof rapid sulfonation of the lignin but, at the same time, it promotesundesirable acid hydrolysis of the carbohydrates in the chips.Furthermore, in such a pulping process, the release of wood acids dur-.ing digestion further lowers the pH of the cooking liquor, increasingits acidity and promoting this acid hydrolysis of carbohydrates. Thisacid hydrolysis has been partially compensated for in such an acidsulfite or bisulfite pulping process by maintaining the digestiontemperature to a level of below approximately 145 to 150 C. While such alow digestion temperature suppresses acid hydrolysis to an extent, it,at the same time, retards the rate of lignin sulfonation and greatlyincreases the time of digestion required to obtain the desired degree ofdelignification and pulping. In addition, in such an acid sulfitepulping process, there is a release of substantial amounts of gaseoussulfur dioxide during digestion which gaseous sulfur dioxide must beremoved and recovered for reuse by a complicated and expensive processin the cooking liquor manufacturing plant.

In conventional neutral or alkaline sulfite pulping processes, thecommon practice is to utilize a cooking liquor having an initial pHwithin the range of approximately 9.0 to 10.0 or even higher to providefor neutralization of the wood acids formed during the digestion and toobtain a digestion pH and/ or terminal pH of 7.0 or higher. The use ofsuch a high initial pH for the cooking liquor has many disadvantages inthat it produces alkaline degradation of certain short chaincarbohydrates (hemi-celluloses) in the wood chips and retards ligninsulfonation increasing the digestion time and/ or requiring a highercooking temperature. Furthermore, the use of cooking liquors containingmagnesium, calcium and other similar insoluble bases for such neutral oralkaline sulfite pulping processes is restricted even to the extent ofeliminating for all practical purposes the use of at least one of suchbases (calcium) because of the tendency of these bases to form insolublemonosulfites which is generally referred to as liming out under alkalineor near alkaline conditions. For instance, using present sulfite pulpingpractices, precipitation of insoluble monosulfites when using thesebases can be avoided only when the ambient liquor pH during the initialdigestion period does not exceed a maximum of approximately 6.0 formagnesium base liquors and even lower (1.5 to 2.0) for calcium baseliquors. Present-day practices used to obtain neutral or alkalinesulfite pulping with liquor containing these insoluble bases includeeither the injection of a suitable alkali dur- "ice ing digestion or thedraining of the acid liquor of the initial digestion stage andsubsequent introduction of an alkaline liquor for a subsequent digestionstage. Both of these practices have the serious disadvantage that theadded alkali must migrate by the slow process of diffusion and liquidexchange into the wood chips which are saturated within an acid liquorpermitting acid hydrolysis of the carbohydrates in the wood chips tooccur while this slow migration is taking place. The practice ofutilizing an alkaline liquor for a second digestion stage has theadditional disadvantage that acid hydrolysis of carbohy drates will haveoccurred before the introduction of the alkaline liquor.

Accordingly, a primary object of this invention is to provide a new andnovel sulfite pulping process which permits the liquor pH during pulpingto be closely controlled whereby optimum pulping conditions may beobtained during the pulping of a wide variety of cellulosic materialsusing a wide variety of liquor bases.

Another object of this invention is to provide a new and novel sulfitepulping process which eliminates virtually all of the undesirableeffects of wood acids bydrolytically released from the pulped materialduring the process.

Still another object of this invention is to provide a new and novelsulfite pulping process which permits pulping at either a substantiallyneutral or even slightly alkaline terminal pH to be obtained withoutalkaline degradation of certain short chain carbohydrates at the startof the digestion, and at an initial pH sufiiciently low to permit arapid rate of lignin sulfonation.

A further object of this invention is to provide a new and novel sulfitepulping process for pulping a wide variety of cellulosic raw materialsutilizing a wide variety of cooking liquors without risk ofprecipitation of insoluble monosulfites from insoluble base liquorscontaining calcium, magnesium, and the like.

Still another object of this invention is to provide a new and novelsulfite pulping process that permits the maintaining of an essentiallyneutral liquor pH throughout the entire digestion and thereby eliminatesthe necessity for the high initial pH with attendant alkalinedegradation of carbohydrates that is characteristic of conventionalneutral sulfite digestions.

A further object of this invention is to provide a new and novel sulfitepulping process which permits the use of a low initial liquor pH for anacid sulfite or bisulfite process as Well as neutral or alkaline sulfiteprocess with substantial elimination of acid hydrolysis of carbohydratesin the pulped material and which permits the conversion of sulfurdioxide liberated during an acid sulfite or bisulfite pulping processinto non-gaseous sulfur compounds.

A still further object of this invention is to provide a new and novelsulfite pulping process in which the initial pH of the cooking liquorcan be varied throughout a wide range from a low pH to substantiallyneutral pH and the digestion and/or terminal pH varied to any valuewithin a range between the initial pH and substantially neutral pH oreven higher with an accelerated digestion time, with virtually no acidhydrolysis of carbohydrates and without the time consuming and expensivesteps of injecting additional alkali during digestion or the use ofmultiplicity of digestion stages.

Still another object of this invention is to provide a new and novelsulfite pulping process which permits the use of sulfurous acid as acooking liquor to obtain rapid lignin sulfonation and a terminal pH ator near neutrality without the customary lignin resinification, chipburning and acid degradation of carbohydrates. I

Other objects and advantages of the invention will become apparent fromthe following description.

The objects stated above and other related objects in this invention areaccomplished by reacting a selected cellulosic raw material such as Woodchips with a sulfite cooking liquor having an initial pH selected inaccordance with the particular type of sulfite pulping process employed.The cooking liquor may have any suitable base such as magnesium,calcium, sodium and even ammonia. An alkali liberating compound such asurea is added to the cooking liquor in an amount which is calculated toproduce a predetermined terminal pH in the pulped mixture at thecompletion of the digestion. Chips are digested at a selectedtemperature which is no lower than the temperature at which ammonia isliberated from the urea whereby the wood acids hydrolytically releasedduring the digestion are neutralized by the ammonia to maintain aselected digestion pH and/or to produce a desired terminal pH in thepulped mixture at the completion of the digestion.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation may be best understood by reference to the followingdescription.

As specifically illustrative of the invention, the alkali liberatingconstituent of the sulfite cooking liquor is urea and urea only will bereferred to in the examples to follow. It should be understood, however,that while the outstanding results of the invention are obtained withthe use of urea, certain other alkali liberating compounds may beemployed such as biuret.

In the chemical reaction which occurs in the practice of the invention,the urea constituent of the cooking liquor is converted to ammonia andcarbon dioxide at temperatures within the range of approximately 110 C.to 170 C. in accordance with the following reactions:

During sulfite pulping process, wood acids such as formic and aceticacids are hydrolytically released from the cellulosic raw material suchas wood chips which are neutralized by the ammonia formed in reactionsNos. 1, 2 by the following reaction:

(3) RCOOH+NH RCOONH Taking reactions 1-3 together, the neutralization ofwood acids by urea can be summarized by the following reaction:

As previously referred to, sulfur dioxide is generated during an acidsulfite or bisulfite pulping process and the neutralization of sulfurdioxide by urea to form ammonium sulfite can be summarized as follows:

In general, the amount of urea combined with the cooking liquor in thesulfite pulping process of the invention is selected in accordance withthe amount of wood acids which are to be neutralized in order to arriveat a desired terminal pH in the pulped mixture. More specifically, theamount of cellulosic raw material to be pulped, the constituents andamount of cooking liquor employed as well as the initial, digestion andterminal pH of the liquor are all factors which must be taken intoconsideration when determining the amount of urea to be added.Furthermore, when using a cooking liquor having as an active ingredientan alkali monosulfite, it is well known that one half of this alkalimonosulfite or combined alkali is available for the neutralization ofthe wood acids released during the digestion. The reaction by which thealkali monosulfite is made available for wood acid neutralization isbest illustrated as follows:

Thus, in determining the total amount of alkali needed to obtain adesired terminal pH in the pulped mixture, only that amount of urea needbe added to the cooking liquor which in combination with one half of thealkali combined as monosulfite, if such is present as a constituent,will produce the desired neutralization of the woods acids producedduring pulping to the desired degree.

In order to clearly illustrate the outstanding results of the invention,various examples were conducted from which handsheets were preparedutilizing both the prior art practices as well as the process of theinvention. In all of these examples with the exception of one whereinpine chips were employed, wood chips comprising a mixture of black gum,sweet gum, tupelo and red maple were digested in a digester using asulfite cooking liquor having the indicated constituents.

In carrying out the various digestion processes, the chips, which werecarefully measured to a weight of 7,000 grams based on bone dry woodsubstance, were placed in the digester and pre-steamed. Then sulfitecooking liquor was pumped into the digester and continuously circulatedthrough the chips. The liquor was brought to the temperature indicatedin each of the examples within a period of 120 minutes and thistemperature, with constant circulation of the liquor through the chips,was maintained for the digestion time indicated in each example. At theend of the digestion, the liquor was removed, the cooked chips weighed,and their bone-dry fiber content was determined to establish yield. Thechips were then given a preliminary refining in a laboratory disc milland a sufficient quantity of the resulting prerefined chips wasintroduced into a standard Valley Beater to constitute a bone dry fibercharge of 454 grams. The total charge of the beater was then adjusted bythe addition of water to 22,700 grams or a slurry of 2% consistency. Theinitial Schopper-Riegler freeness was determined and 5.5 kilograms ofweight were placed on the bedplate arm. Beating of the charge was thencarried out and pulp samples of standard Schopper- Riegler freeness of750 cc., 500 cc., 250 cc., and 150 cc. were caught and the time toobtain each of these freenesses was measured. Handsheets having a basicweight of pounds per 3,000 square feet were prepared from the pulps atthese standard freenesses. After a conditioning period of 24 hours at50% relative humidity and 73 F., the various handsheets were subjectedto a GE. Brightness, 6" Ring Crush (Riehle), Tear, Mullen and ZetaTensile Tests.

The following two examples were conducted to illustrate present daysodium base sulfite pulping processes:

Example I Schopper-Riegler Freencss, cc .i 750 500 250 150 G.E.Brightness, percent 42 0 Minutes Beating i. 14 39 51 Density, Grams/cc0. 532 0. 647 0. 728 0. 775 6 Ring Crush, Pounds i 68 93 107 112 Tear,Grams 9t) 95 91 76 Mullen, Pounds 54 88 111 114 Zeta Tensile, Pounds 37115 188 H...

Example 2 As an example of a conventional high yield sodium basebisulfite digestion process, the wood chips were Schopper-RieglerFreeness, cc 750 500 250 150 G.E. Brightness, percent 380 MinutesBeating 8 42 63 Density, Grams/cc O 490 625 0.720 0.808 6 Ring Crush,Pounds 60 88 104 117 Tear, Grams 94 101 107 98 Mullen, Pounds 50 74 7597 Zeta Tensile, Pounds 43 92 125 The tollowing three examples wereconducted to illustrate the novel process of this invention as appliedto sodium base sulfite pulping:

Example 3 This example is an illustration of the sulfiite pulpingprocess of the invention utilizing a sodium base bisulfite liquor. Theactive ingredients of the cooking liquor were 7 16 grams of sodiumbisulfite and 294 grams of urea at an initial pH of 3.9. The digestionwas maintained at 170 C. for 150 minutes. The terminal pH was 7.0, andthe yield, based on the original bone dry wood, was 74.5%. The handsheetproperties were as tfollows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, percent.37 Minutes Beating 11 24 4 55 Density, Grams/cc 0.525 0 620 0.732 0.7766 Ring Crush, Pounds 65 77 86 92 Tear, Grams 118 144 128 118 Mullen,Pounds 62 87 98 104 Zeta Tensile, Pounds 56 97 154 Example 4Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, percent41 Minutes Beating 17 44 61 Density, Grams/cc.-." 0.610 0. 705 0.7300.795 6 Ring Crush, Pounds 73 92 111 100 Tear, Grams 114 112 103 90Mullen, Pounds 77 98 114 118 Zeta Tensile, Pounds 90 153 181 Example 5This example is similar to Example 4 but with reduced amounts of pulpingchemicals. The active ingredients of the cooking liquor were 378 gramsof sodium sulfite, 308 grams of sodium bisulfite, and 150 grams of urea,with an initial pH of 7.5. The digestion was maintained at 170 C. for100 minutes. The terminal pH was 7.4, and the yield, based on theorginal bone dry wood, was 76.6%. The handsheet properties were asfollows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, percent26 Minutes Beating 14 28 50 74 Density, Grams/em 0.575 0.675 0.732 0.775 6" Ring Crush, Pounds. 71 96 96 100 Tear, Grams 91 118 115 92Mullen, Pounds 61 85 108 113 Zeta Tensile, Pounds 76 128 161 Thetfiollowing example was conducted to illustrate a present day magnesiumbase bisultfite pulping process:

Example 6 This example demonstrates the results obtained with aconventional high yield magnesium base bisulfite digestion of hardwood.The active ingredient of the cooking liquor was 620 grams of magnesiumbisulfite, and the initial pH was 4.9. The digestion was maintained at150 C. for 180 minutes. The terminal pH was 3.9, and the yield, based onthe original bone dry wood, was 6 8.7%. The handsheet properties were asfollows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, pereent37 Minutes Beating 15 39 Density, Grams/ce 0.625 0.740 0.811 0.870 6'Ring Crush, Pounds 81 94 113 Tear, Grams 122 113 114 106 Mullen, P0unds76 87 102 107 Zeta Tensile, Pounds 88 147 200 The following threeexamples were conducted to illustrate the novel process of thisinvention as applied to magnesium base bisulfite pulping? Example 7 Thisexample is an illustration of the sulfite pulping process of theinvention utilizing a magnesium base bisulfite liquor which isterminated at a relatively high pH. The active ingredients of thecooking liquor were 620 grams of magnesium bisulfite and 290 grams ofurea, with an initial pH of 4.7. The digestion was maintained at 170 C.for 105 minutes. The terminal pH was 6.1, and the yield, based on theoriginal bone dry wood, was 71.5%. At one time during the digestion a pHof 7.1 was observed with no evidence of liming out. The handsheetproperties were as follows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, Percent83 Minutes Beating 9 33 58 85 Density, Grams/cc 0. 695 0. 745 0.8200.862 6 Ring Crush, Pounds. 77 92 92 101 Tear, Grams" 126 123 100Mullen, Pounds 95 109 128 111 Zeta Tensile, Poun 85 134 200 Example 8erties were as follows:

Schopper-Riegler Freeness, cc 750 500 250 G.E. Brightness, Percent 21Minutes Beating 5 22 40 68 Density, Grams/cc. 0.560 0.660 0.763 0.830 6"Ring Crush, Pou 50 68 87 113 Tear, Grams... 87 120 97 95 Mullen, Pounds84 92 104 110 Zeta Tensile, Pound 55 131 171 Example 9 This example isan illustration of a magnesium base bisulifite pulping process inaccordance with the invention on which the pulped cellul-osic materialis southern pine chips. The active ingredients of the cooking liquorwere 930 grams of magnesium bisulfite and-360 grams of urea, with aninitial pH of 4.3. The digestion was maintained at C. for 210 minutes.No evidence of liming out was observed, although the terminal pH was7.3. The yield, based on the original bone dry wood, was 84.3%. Littleor no evidence of objectionable pitch or tacky sub- 67.3%. There was noevidence of burning or lignin resinification. Handsheet properties wereas follows:

stances was observed in the pulp. Handsheet properties Schopper-Riegler13110110557 66 750 509 250 150 were as follows: 5

G.E.Brightn ess, Percent 32 .i Schopper-RieglerFreenescc 750 500 250 1591B r?$ ;j;jj 0. 544 0.715 0.155 0.770 0" Ring Crush, Pound 56 fig fit FPercent 8 5 g l i ifiiien i t i u i'ias 5 J3 120 mu e" ea 111g...

. 0.610 (L640 0750 0770 Zeta Tens1le,Poun 51 140 14a 52 74 69 69 32 ,3332 8% In order to provide a clear comparison of the results ZetaTensile. Pound 77 90 125 obtained in the various examples previouslyreferred to and thereby contrast the novel results obtained with the Thefollowing example was conducted to illustrate the process of theinvention with the prior art, a tabulation novel pulping process of thisinvention as applied to 6211- of the results of Examples 1-11 have beencomplled 1n inm b a id lfite pulpin the table below ident1fied as TableI. Only the test re- Exam 10 sults obtained at Schopper RieglerFreenesses of 750 cc.

p and 500 cc. are included in this table, as the general com- The activeingredients of the cooking liquor were 202 mercial use of these SchopperRiegler Freenesses is congrams of calcium bisulfiate, 242 grams of freeS0 and sidered to provide sufiicient comparison of the test results.

TABLE I Example No 1 2 3 4 5 6 7 8 9 10 11 Type of Wood Pulped Pine ase(I) Mg Mg Mg Mg Ca None Sodium Sulfite, gms 882 0 0 590 378 0 0 0 U 0 0Bisulfite,gms 0 693 716 198 308 620 620 465 930 202 0 Free SOe,gms 0 O 00 0 0 0 0 0 242 427 Bufler Used 0 Urea Urea Urea 0 Urea Urea Urea UreaUrea Amount of Bnner, gms. 159 0 294 143 150 0 290 270 360 470 550InitielDigestionpH- 10.6 5.3 3.9 7.0 7.5 4.9 4.7 4.6 4.3 1.9 1.8TerminalDigestion pH. 7.4 4.7 7.0 7.5 7.4 3.9 6.1 6.2 7.3 7.2 7.15Cooking Temp, 0.. 170 150 170 170 170 150 170 170 170 170 170 CookingTime, Mins" 180 250 150 150 100 180 105 90 210 60 Yield, Percent 75.072.3 74.5 72.2 76.6 68.7 71.5 74.9 84.3 75.8 72.2 G.E. Brightness,Percent. 42.0 38.0 37.0 41.0 26.0 37.0 33.0 21.0 32.0 29.5 32.0 750sRDens'y 0.532 0.490 0.525 0.610 0.575 0.625 0.695 0.560 0.610 0.5340.544 6" Ring Crush. 68 73 71 81 77 60 52 53 56 Tear, gms 90 94 118 11491 122 126 87 183 106 93 Mullen, lbs 54 56 62 77 61 76 95 84 83 57 52Zeta Tensile, lbs 37 43 56 90 76 88 85 55 77 58 51 500 SR-Density. 0.6470.625 0.620 0.705 0.675 0.740 0.745 0.660 0.640 0.589 6.715 6 RingCrush. 93 88 77 92 96 94 92 68 74 90 'leur,gms 95 101 144 112 118 113135 120 170 128 113 Mullen, lbs... 88 74 87 98 87 109 92 98 73 03 ZetaTensile, lbs 115 92 97 153 128 147 134 131 75 145 1 Hardwoods. 2 Sodium.NBzCO 470 grams of urea. The initial pH was 1.9. The time con- As hasbeen referred to above, Examples 1, 2 are consumed in heating the chargeto 170 C. was extended to ventional neutral sulfite and sodium bisulfitepulping proc 330 minutes to avoid possible resinification of the lignmesses using a sodium base with Examples No. 3, 4, and 5 by the highlyacid liquor. The digestion was maintained carried out in accordance withthe novel process of the at 170 C. for 60 minutes. The terminal pH was7.2, and 50 invention utilizing a sodium base. Example No. 6 is a conatno time during the digestion was there visible evidence ventionalpulping process using a magnesium base and of liming out. The yield,based on the original bone dry Examples N0. 7, 8 are magnesium basesulfite pulping wood, was 75.8%, with no evidence of burning.Handprocesses showing the novel results of the process of the sheetproperties were as follows: invention as applied to a magnesium baseprocess. Ex-

55 ample N0. 9 illustrates the results of magnesium baseSchopper-RieglerFreeness,cc 756 500 250 150 pulping utilizing pine asthe pulped material. Example No. 10 is a calcium base sulfite pulpingprocess and Ex- (1.12. Brightness, Percent 29.5 ample No. 11 is afurther embodiment of the invention Minutes Beating 16 39 68 100 f so thK Density, Grams/cc 0.534 0.589 0.668 0.877 1 g Tee 2 as 6 8 i 6" RingCrush, Pounds 53 80 85 8 0 One of the outstandlng features of theinvention is the Tear, Grams 106 128 128 121 b r Mullen, pounds 57 73109 Sll stantlal red ction in cooking time over prior art proc- ZetaTensile. Pounds 58 75 135 esses using the process of the invention, Thusc n id bl cost savings are effected by reduced production time while Thefollowing example was conducted to illustrate the simultaneouslyimproving the quality of the product. In novel pulping process of thisinvention as applied to sul- 65 the sodium base pulping processes of theinvention, cookfurous acid pulping. mg times of 150 minutes for Examples3, 4, and minutes for Example No. 5 compare quite favorably with Example11 I 180 mmutes and 250 minutes for prior art Examples The activeingredients of the cookmg liquor were 427 No. 1,2. The magnesium basesulfite pulping processes grams of free S0 and 550 grams of urea. The1n1t1al pH 70 0f the 1nvent1on show cooking times of and 90 minwas 1.8.The time required to bring the change to 170 C. was extended to minutesto avoid possible burning by the highly acid liquor. The change wasmaintained at C. for 45 minutes. The terminal pH was 7.15 and the yield,based on the original bone dry wood, was

utes for Examples 7, 8 respectively as compared with the conventionalmagnesium pulping process in Example No. 6 of minutes.

At the Schopper Riegler Freeness of 750 cc., the sodium base ExamplesNo. 3, 4, and 5 show substantial increases in 6" Ring Crush, Tear,Mullen and Zeta Tensile over the prior art sodium base process ofExamples No. 1, 2. At Schopper Riegler Freeness of 500 cc.,substantially the same improved results were obtained with unusuallyhigh Tear in Example No. 3 and Zeta Tensile in Example No. 4.

In the magnesium base sulfite pulping Examples No. 7, 8 and at 750 cc.Schopper-Riegler freeness, the 6" Ring Crush, Tear and Zeta Tensilecompare favorably with the prior art Example No. 6 and with asubstantial improvement in Mullen being obtained. At Schopper-Rieglerfreeness of 500 cc., essentially the same results in 6" Ring Crush, ZetaTensile and Tear as the prior art were obtained but with a considerableimprovement in Mullen. A significantly high Tear was obtained in ExampleNo. 7 as compared with prior art Example No. 6 at 500 cc. Schopper. Whatis particularly unusual regarding the test results of Examples Nos. 7, 8is that although a terminal pH approaching neutral was involvedincluding an even higher pH at one time during digestion, there is noevidence of liming out or precipitation of monosulfites such as iscommon in the present day magnesium base sulfite pulping processes.

Example No. 9 was included to show the advantageous use of the processof the invention in the magnesium base sulfite pulping of pine. Testresults shown for Example No. 9 compare quite favorably with the testresults of the conventional magnesium base pulping of hardwood inExample No. 6 with an unusually high Tear and Mullen being obtained. Asin Examples Nos. 7, 8, there was no evidence of liming out in ExampleNo. 9 and the process of the invention apparently prevented the formingof objectionable pitch or other tacky substances in the pulp.

Example No. 10 shows that the process of the invention may be used toaccomplish what is virtually impossible with prior art practices in thatwith an initial pH of 1.9 in a calcium base sulfite pulping ofhardwoods, a terminal pH of 7.2 was reached in a cooking time of only 60minutes without any evidence of liming out. The test results show theproducton of a pulp of more than adequate strength at 750 cc. and 500cc. Schopper-Riegler freeness.

Example No. 11 shows another unusual result of the process of theinvention wherein a highly acid free S was used as a cooking liquorbuttered with urea in accordance with the invention and at an initial pHof 1.8 and a terminal pH of 7.15. The common drawbacks of S0 pulpingsuch as ligin resinification, chip burning, and acid degradation ofcarbohydrates did not appear and a pulp was obtained with unusually goodtest results.

In view of the foregoing examples in which the effectiveness of a widevariety of sulfite bases or even sulfurous acid as pulping agents hasbeen demonstrated, it logically follows that ammonia or lessconventional bases may be employed without departing from the spirit ofthis invention.

The results obtained with the novel process of the invention in whichthe cooking liquor is buffered with an alkali liberating compound showthat a pulp may be produced utilizing any of a wide selection of basesincluding sodium, magnesium, and calcium so as to produce a pulp atleast equal to and in many cases far superior to sulfite pulps producedutilizing prior art processes. The well known tests of 6" Ring Crush,Tear, Mullen and Zeta Tensile which were performed on handsheetsproduced in accordance with the invention are considered to clearlyreveal all of the desired strength factors of paper produced from pulp.In addition to the production of superior pulp, the cooking time in eachexample in the pulping of hardwoods was reduced considerably over thatof the prior art. What is of particular significance is that the processof the invention permits the pH of the cooking liquor to be controlledto a very close degree throughout the entire pulping process andeliminates the severe drawbacks of alkaline degradation and acidhydrolysis heretofore encountered with the necessary use of cookingliquors at a pH Within the alkaline range of 10 and above or in the acidrange for prolonged periods. Of additional significance is theelimination of the precipitation of monosulfites when magnesium orcalcium bases are utilized as Well as the conversion of sulfur dioxideto ammonium sulfite during acid sulfite pulping using the process of theinvention which eliminates an additional problem of the prior artprocesses where gaseous S0 was formed requiring its disposal.

While there has been described what at present is considered to be thepreferred embodiment of the invention, it will be understood by thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention and, therefore, it is theaim of the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

Having thus described the invention, what is claimed is:

1. A sulfite pulping process comprising .the steps of mixing cellulosicraw material with a sulfite cookingliquor having urea as a constituentin an amount sufficient to produce a predetermined terminal pH in themixture, reacting said liquor and said cellulosic raw material at atemperature no lower than the temperature at which ammonia is formedfrom said urea for neutralizing the wood acids hydrolytically releasedduring said reaction and continuing said reaction to completion at saidpredetermined terminal pH to form for processing into fibrous sheetmaterial.

2. A sulfite pulping process comprising the steps of, mixing cellulosicraw material with sulfite cooking liquor having urea as a constituent inan amount sufficient to produce a substantially neutral terminal pH inthe mixture, digesting said cellulosic raw material by reacting saidliquor and said cellulosic raw material at a temperature no lower thanthe temperature at which ammonia is formed from said urea forneutralizing the wood acids hydrolytically released during the saiddigestion, controlling the neutralization of said wood acids by saidammonia to maintain a substantially neutral digestion pH, and continuingsaid digestion to completion at said substantially neutral terminal pHto form pulp for processing into fibrous sheet material.

3. A sulfite pulping process comprising the steps of, mixing cellulosicraw material with a sodium base sulfite cooking liquor having urea as aconstituent in an amount sufficient to produce a predetermined terminalpH in the mixture, digesting said cellulosic raw material by reactingsaid liquor and said cellulosic raw material at a temperature no lowerthan the temperature at which ammonia is formed from said urea forneutralizing the Wood acids hydrolytically released during saiddigestion, controlling the neutralization of said wood acids by saidammonia to maintain a selected digestion pH and continuing saiddigestion to completion at said predetermined terminal pH to form pulpfor processing int-o fibrous sheet material.

4. A sulfite pulping process comprising the steps of, mixing cellulosicraw material with a magnesium base sulfite cooking liquor having urea asa constituent in an amount sufficient to produce a predeterminedterminal pH in the mixture, digesting said cellulosic raw material byreacting said liquor and said cellulosic raw material at a temperatureno lower than the temperature at which ammonia is formed from said ureafor neutralizing the Wood acids hydrolytically released during saiddigestion, controlling the neutralization of said Wood acids by saidammonia to maintain a selected digestion pH and continuing saiddigestionto completion at said predetermined terminal pH to form pulpfor processing into fibrous sheet material.

5. A sulfite pulping process comprising the steps of, mixing cellulosicraw material with a calcium base sulfite cooking liquor having urea as aconstituent in an amount sufficient to produce a predetermined terminalpH in the mixture, digesting said cellulosic raw material by reactingsaid liquor and said cellulosic raw material at a temperature no lowerthan the temperature at which ammonia is formed from said urea forneutralizing the wood acids hydrolytically released during saiddigestion, controlling the neutralization of said wood acids by saidammonia to maintain a selected digestion pH and continuing saiddigestion to completion at said predetermined terminal pH to form pulpfor processing into fibrous sheet material.

6. A sulfite pulping process comprising the steps of, mixing cellulosicraw material with a sulfite cooking liquor having a base selected fromthe group consisting of sodium, magnesium and calcium and having urea asa constituent in an amount sufiicient to produce a predeterminedterminal pH in the mixture, digesting said cellulosic raw material byreacting said liquor and said cellulosic raw material at a temperatureno lower than the temperature at which ammonia is formed from said ureafor neutralizing the Wood acids hydrolytically released during saiddigestion, controlling the neutralization of said wood acids by saidammonia to maintain a selected digestion pH and continuing saiddigestion to completion at said predetermined terminal pH to form pulpfor processing into fibrous sheet material.

7. A sulfite pulping process comprising the steps of, mixing cellulosicraw material with a solution of free sulfur dioxide as a cooking liquorhaving urea as a constituent in an amount sufiicient to produce apredetermined terminal pH in the mixture, reacting said sulfur dioxideand said cellulosic raw material at a temperature no lower than thetemperature at which ammonia is formed from said urea for neutralizingthe wood acids hydrolytically released during said reaction, andcontinuing said reaction to completion at said predetermined terminal pHto form pulp for processing into fibrous sheet material.

References Cited UNITED STATES PATENTS 1,880,046 9/1932 Richter 162832,071,304 2/1937 Hirschkind 162-72 X 2,361,639 10/1944 Loughborough162-72 X 3,161,562 12/1964 Gillaspie 162--72 DONALL H. SYLVESTER,Primary Examiner.

H. CAINE, Assistant Examiner.

1. A SULFITE PULPING PROCESS COMPRISING THE STEPS OF MIXING CLEEULOSICRAW MATERIAL WITH A SULFITE COOKING LIQUOR HAVING UREA AS A CONSTITUENTIN AN AMOUNT SUFFIECIENT TO PRODUCE A PREDETERMINED TERMINAL PH IN THEMIXTURE, REACTING SAID LIQUOR AND SAID CELLULOSIC RAW MATERIAL AT ATEMPERATURE NO LOWER THAN THE TEMPERATURE AT WHICH AMMONIA IS FORMEDFROM SAID UREA FOR NEUTRALIZING THE WOOD ACIDS HYDROLYTICALLY RELEASEDDURING SAID REACTION AND CONTINUING SAID REACTION TO COMPLETION AT SAIDPREDETERMINED TERMINAL PH TO FORM FOR PROCESSING INTO FIBROUS SHEETMATERIAL.